The effects of low doses of ionizing radiations on mammalian cells will be investigated and the data analyzed and interpreted within the framework of a biophysical theory. A variety of biological endpoints will be observed from chromosome aberrations and sister chromatid exchanges, through cell lethality, to the production of oncogenic transformations. Large scale experiments are performed so that observations can be made at doses down to 1 rad. The analysis of the data in the light of theory is designed to permit extrapolations to doses too low for direct experimental observation. Modification of dose response relationships by environmental factors such as oxygen tension and temperature, and the concomitant use of chemical and pharmacological agents are an important objective of the investigation. A variety of radiations are to be studied, including x-rays, gamma-rays, neutrons of a wide range of energies, protons, deuterons, alpha particles and high energy heavy ions. In addition to intrinsic scientific importance, the results of this research have pragmatic value in radiation protection and radiation oncology.